In internal combustion engines functioning with externally supplied ignition, it is known to control the instant of ignition in a closed-loop fasion in accordance with the occurrence of the peak pressure relative to the occurrence of top dead center during the combustion process. An optimal set-point value representative of the angular crankshaft position at which the peak pressure is supposed to occur in the combustion chamber is gnerated, and the instant of ignition is appropriately corrected when there is a deviation from this set-point value (See Powell, "Closed Loop Control of Spark Timing", Automobile Engine Control Symposium, April 1976).
It is also known, from German Offenlegungsschrift (laid-open application) No. 24 49 836, to control the composition of the operational mixture in a closed-loop fashion in terms of the fuel, air, or exhaust gas component in accordance with the fluctuations of the combustion chamber pressure at predetermined crankshaft angles in sequential work cycles. According to the teaching of this German application, the engine roughness is evaluated as a control variable in the region of the lean running limit of the engine, where, because of the irregularity of ignition, more or less severe fluctuations in the attainable combustion chamber pressure occur. In the described method, a representative signal of the combustion chamber pressure is integrated over the work cycle and an interrogation is performed at predetermined angles of the crankshaft rotation. The degree of fluctuations serves as a standard for the approach to the lean running limit, while a predetermined set-point value represents the threshold values for the closeness of permissible approach to the lean running limit. Because a substantial increase in the average combustion chamber pressures first occurs at relatively severe leaning, this method is usable only in the case of an operational mixture which is to be severely leaned down.